Assessment of two-stage hyper- and thermophilic anaerobic co-digestion of briquetted wheat straw and liquid fraction of digestate

Abstract

Due to the high resistance of lignocellulosic biomass to anaerobic digestion, the application of an appropriate pre-treatment method is an unavoidable step before subjecting the material to anaerobic digestion (AD). Thus, this study aims to assess a two-stage anaerobic co-digestion of briquetted wheat straw and liquid fraction of digestate under hyper- and thermophilic conditions (65 and 55˚C). Hyperthermophilic AD was applied in the first stage as a pre-digestion step to investigate the impact of increased temperature on the degradation level of lignocellulosic biomass throughout the process. After operating 10 hydraulic retention times the average methane yield was determined to be 248 (±40) L/kgVS with 4 days retention time at hyperthermophilic conditions followed by 14 days thermophilic anaerobic digestion. The methane yield contribution from the briquetted wheat straw was found to be 277 (±26) L/kgVS by subtracting the yield from the liquid fraction of digestate. A mass balance was performed to evaluate the degradability of lignocellulose throughout the process. Hemicellulose and cellulose degradation rates were ∼ 68.7 %, and 68.0 %, respectively for both digestion steps. The experiment reported that two-stage anaerobic co-digestion under hyper- and thermophilic conditions is an attractive strategy for maximizing energy yield from straw by enhancing the lignocellulose degradation and might pose an alternative pre-treatment method of agricultural wastes.